The invention relates to a method for the thermal treatment of mealy raw materials, particularly in the reduction of cement clinker from raw meal, in which the raw meal is thermally treated in a combustion process by preheating, calcination, sintering and cooling, and the exhaust gas stream of the sintering stage (rotary kiln) and an exhaust airstream (tertiary air) of the cooling stage (clinker cooler) are used in such a way for the calcination of the raw meal in the calcination stage that fuel is introduced into both the rotary kiln exhaust gas channel and the tertiary air channel; the two channels are merged; and the gas-solid suspension contained therein is fed into the lowermost cyclone of the cyclone suspension type pre-heater system for the purpose of separating the calcinated raw meal from the gas stream. In addition, the invention relates to an apparatus for carrying out the method.
Methods and apparatuses of this typexe2x80x94which are known from EP-B-0 526 770, for examplexe2x80x94must be in the position to generate a high-grade calcinated raw meal in the calcination stage, which is double-fired and which is connected to the rotary tubular kiln upstream, prior to transport into the rotary tubular kiln. At the same time, the builders and operators of the cement clinker production lines are confronted with ever stricter demands for low emission values of pollutants such as NOx and CO.
The builders and operators of cement clinker production lines are also confronted with the problem of using an optimally high proportion of lumpy fuels such as unchipped used tires and/or other lumpy and potentially inert combustibles which are difficult to ignite, or respectively, difficult to burnxe2x80x94xe2x80x94which are referred to overall as secondary fuelsxe2x80x94xe2x80x94for the fuels that are used in the calcination stage, and of expediently disposing of such substances in this way for the calcination of the cement meal. Developments in the usage of used tires for the purpose of using their combustion energy are directed more and more to the utilization of large used tires, specifically truck tires, in the process without first chipping them into smaller lumpy particles by means of expensive comminution work.
It is known from EP-A-0 803 693 to insert whole automobile tires in a cement clinker production line into the rotary tubular kiln exhaust gas channel, or respectively, into the input shaft to the rotary tubular kiln in a recumbent position, as a result of which the used tires should gasify and burn in the input shaft, and the residual substances should drop down into the rotary tubular kiln. In this solution, it is disadvantageous that when a tire is pushed into the input shaft, the cross-section of the shaft is narrowed appreciably, which leads to a rise in pressure loss and to an unstable operation of the system. Furthermore, a table for bearing the used tires that are inserted, which is built into the input shaft as a grate or a solid plate, interferes with the normal process quite significantly, owing to the danger of sediment or crust formations. In addition, when a used tire is inserted, a concentrated energy addition suddenly arises in the input shaft, or respectively, in the area of the rotary tubular kiln input. Consequently, there is usually not enough oxygen remaining for the pyrolysis, vaporization, and combustion processes that commence immediately subsequent to this, so that undesirable CO surges, or respectively, CO peaks occur in the rotary tubular kiln exhaust gas, which can then lead to the deactivation of the electrostatic dust trap (electrofilter) downstream if the CO peaks exceed a defined limit value. The sudden presentation of energy in the process is usually undesirable, because the necessary streams of raw material mass are not spontaneously available for converting the heat into calcination work.
In the usage of used tires in cement clinker production, it is also known (EP-B-0 439 824) to burn the used tires in the rotary kiln input chamber, or respectively, in the stock input area of the rotary tubular kiln, with an oxygen surplus. Here, the excess oxygen must be drawn through the whole rotary tubular kiln in the form of excess secondary air. Undesirable CO surges occur here also, which limit the amount of the charge of used tires.
Finally, the journal xe2x80x9cZement-Kalk-Gipsxe2x80x9d (1/1999) has made known a calcination system having a separate secondary reactor, which is called a shaft vaporizer, in which whole used tires are vaporized, the resulting gas product being fed to the calcinator in a separate conduit. The residual matter of the used tires that have been vaporized in the shaft vaporizer should be transported into the input shaft of the rotary kiln by a mechanical push mechanism. In this shaft vaporizer, the inserted tires fall from above onto a non-gas-permeable table, and the tertiary air substream that is used as vaporization medium is likewise fed into the shaft vaporizer (reactor) from above. By virtue of the direct feeding of the pyrolysis gas from the shaft vaporizer into the calcinator, the actual calcination process, which occurs only in the rotary kiln exhaust gas line in the known system, is influenced by a vaporization or pyrolysis process, which may undergo sharp fluctuations. Greater and greater amounts of secondary fuel are located in the reactor. This necessitates a special safety mechanism in order to be able to bleed off the pyrolysis gas via the roof in accordance with the corresponding safety specifications in case of a disruption of the overall system.
It is an object of the invention to create, in cement clinker production lines of the above described type, a double fired calcinator that is connected upstream to the rotary tubular kiln, in which calcinator particularly lumpy combustibles, such as unchipped used tires and/or other lumpy secondary combustibles, can be expediently disposed of by pyrolysis xe2x80x94or optionally by combustionxe2x80x94for the calcination of the cement raw meal, without the pyrolysis process, or respectively, the combustion process, interfering with the calcination process, and without giving rise to pulsations in the overall system, while simultaneously ensuring a residual burnout of CO surges and also ensuring an exhaust gas having low NOx emissions.
It is characteristic of the inventive calcination stage of a cement clinker production line that a lumpy combustible such as a whole used tire is slid onto a displacement mechanism such as a grate, which is arranged transversely in a tertiary air channel in the area of the junction with the exhaust gas channel of the rotary tubular kiln, on which table the lumpy fuel is pyrolized by means of a tertiary air substream that is diverted from the tertiary air line, and from which table the pyrolysis residue is pushed into the exhaust gas channel of the rotary kiln. The pyrolysis gas is likewise fed into the rotary kiln""s exhaust channel, where it forms a CO-laden reduction zone (CO cloud) for the purpose of reducing the NOx contained in the rotary kiln exhaust.
The tertiary air channel that is provided for the pyrolysis of the lumpy fuel is a pyrolysis channel to the extent that, in the area of its inlet into the rotary kiln exhaust channel, it is constructed as a pyrolysis chamber for the pyrolysis of the lumpy fuel, for instance used tires. In the pyrolysis chamber, the grate is arranged for accepting the lumpy fuel (e.g. used tires) such that, proceeding approximately from the junction region between the rotary kiln""s exhaust gas channel and the pyrolysis channel, it extends transversely over at least part of the cross-section of the of the pyrolysis chamber. Outside the pyrolysis chamber, an apparatus for introducing the lumpy fuel (e.g. used tires) is arranged laterally at the pyrolysis channel.
As lumpy fuel, besides unchipped used tires, other secondary combustibles can be introduced into the pyrolysis channel, or respectively, the pyrolysis chamber, such as compacted residual resources from garbage, plastic material or other combustible substances which have been previously compacted in a separate process outside of the cement clinker production process. The grate, which is penetrated by a territory air substream, for accepting the lumpy fuel in the pyrolysis chamber can be arranged horizontally or at an oblique inclination to the junction area between the exhaust gas channel of the rotary kiln and the pyrolysis channel. The grate advantageously stands in connection with a pushing mechanism for discarding the pyrolysis residue, or respectively, the unburned residual matter, into the exhaust gas channel of the rotary kiln. The grate can also be a traveling grate.
The pyrolysis channel that has the pyrolysis chamber for the pyrolysis of the lumpy fuel is not integrated in the actual calcination channel, which is also penetrated by a tertiary air substream and which has at least one raw meal input and at least one fuel input; rather, it is installed next to this calcination channel. The pyrolysis channel is advantageously arranged between the rotary kiln exhaust channel and the calcination channel, which opens into the exhaust gas channel of the rotary kiln above the junction of the pyrolysis channel and the exhaust gas channel of the rotary kiln. Given this inventive integration of the pyrolysis channel for the lumpy fuel into the calcination stage of a cement clinker production line, the calcination channel, with the raw meal calcination process that transpires there, is not disturbed by the pyrolysis process, even when large obtrusive solids are pyrolized, and so pulsations do not arise in the gas flow of the overall system. The pyrolysis process can advantageously be controlled in a process-dependent manner by means of an adjusting valve that controls the air supply. At the same time, the CO cloud that results from the pyrolysis of the lumpy fuel is an effective medium for reducing NOx in the exhaust gas. Downstream from the junction of the exhaust channel of the rotary kiln and the pyrolysis channel, an additional tertiary air substream empties into the exhaust gas channel of the rotary kiln for the combustion of the CO strands that may have remained outside the reduction zone and/or of other fuel components that have remained unburned, for the purpose of a residual burnout of such residual combustibles.